1. Field of the Invention
The present invention relates to a communication system, particularly to a communication system in which a line between exchange stations is duplicated of a cable line and a radio line.
2. Description of the Related Art
There has heretofore been a communication system in which a telephone line between exchange stations is duplicated of a working cable line and a protection radio line (satellite line) for countermeasures against natural calamities, and the like. Since the satellite line can be used in a limited radio frequency, the number of lines cannot be increased. Therefore, by providing a radio line side with digital circuit multiplication equipment (DCME) defined by ITU-T G. 763, and sending a compressed signal, it is possible to effectively utilize resources of the limited radio frequency.
However, since the communication system in which the line is doubled is not considered in the ITU-T G. 763, a method of transmitting a line switch signal to realize line switch is not defined.
Therefore, in the communication system in which the telephone line between the exchange stations is duplicated of the cable line and radio line, when the DCME is disposed on the radio line side, a transmit-side line and a receiver-side line need to be individually switched.
FIG. 4 is a block diagram showing the constitution of the conventional communication system. Lines 32a connected to an exchanger (exchange station) 31a transmit 2048 kbit/s signals defined by ITU-T G. 732. These lines 32a are connected to a line switch circuit 33a. The line switch circuit 33a connects the line 32a to either line 34a or 35a in accordance with the control of a controller 40.
A multiplexer 36a multiplexes the signals transmitted by the lines 34a. Moreover, upon receiving the multiplexed signal from a cable line 37, the multiplexer 36a demultiplexes this signal into four 2048 kbit/s signals, and outputs these signals to the lines 34a. The multiplexer 36b demultiplexes the multiplexed 8192 kbit/s signal transmitted by the cable line 37 into four 2048 kbit/s signals, and outputs these signals to the lines 34b. The multiplexer 36b multiplexes the 2048 kbit/s signals inputted from the lines 34b to provide a 8192 kbit/s signal, and transmits this multiplexed signal to the cable line 37.
On the other hand, DCME 38a multiplexes the 2048 kbit/s signals transmitted by the lines 35a, and transmits the multiplexed signal to a radio line (satellite line) 39. Moreover, upon receiving the multiplexed signal from the radio line 39, the DCME 38a demultiplexes this signal into four 2048 kbit/s signals, and outputs these signals to the lines 35a. DCME 38b demultiplexes the multiplexed signal transmitted by the radio line 39 into four 2048 kbit/s signals, and outputs these signals to lines 35b. Moreover, the DCME 38b multiplexes the 2048 kbit/s signals inputted from the lines 35b, and transmits the multiplexed signal to the radio line 39. A line switch circuit 33b connects either line 34b or 35b to a line 32b in accordance with the line switch signal outputted from an extractor 42b described later. The lines 32b are connected to an exchanger (exchange station) 31b. 
In the above-described communication system, the controller 40 outputs a line switch signal to an inserter 42a. The inserter 42a inserts the line switch signal into the channel of the line 35a for transmitting a data signal or a phone signal. Moreover, the extractor 42b extracts the line switch signal transmitted from the remote station side from the line 35b. The line switch circuit 33a can be operated in cooperation with the line switch circuit 33b in this manner.
In the above-described conventional communication system, since the channel for transmitting the data signal or the phone signal is used to transmit the line switch signal, and the local station side line switch circuit is operated in cooperation with the remote station side line switch circuit, there is a problem that the transmission efficiency is deteriorated.